The present invention relates to the field of two-way communication systems and more specifically to methods and apparatus for communication between communication units (CU) in a multiple access system. In particular embodiments, the invention relates to mobile telephone data users (cellular and personal communication systems), basic exchange telecommunications radio, wireless data communications, two-way paging and other wireless systems.
Conventional Cellular Systems.
Current cellular mobile telephone systems developed due to a large demand for mobile services that could not be satisfied by earlier systems. Mobile telephone cellular systems based upon time division multiple access (TDMA) protocols “reuse” frequencies within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of RF spectrum available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The mobile networks provide for the hand-off of users from cell to cell and thus typically from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
In addition to providing RF connectivity to users, the base station also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's are used over the covered region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MTSO by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MTSO depends upon the traffic at each base station, the cost of interconnection between the MTSO and the base stations, the topology of the service area and the switching capacity of the MTSO.
A handoff between base stations occurs, for example, when a mobile user travels from one cell to an adjacent cell. Handoffs are also performed to relieve the load on a base station that has exhausted its traffic-handling capacity or to improve a call undergoing poor quality communication. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and have not yet been established with the second cell.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes rapidly as the distance between the transmitter and receiver increases. Conventional frequency reuse techniques rely upon this rapid fall-off in power as a function of distance. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell such that the reuse site signals do not significantly interfere with the carrier in the original cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details of specific cellular standards.
TDMA Conventional Cellular Architectures.
In TDMA systems, time is divided into time slots of a specified duration. Time slots are grouped into frames, and the homologous time slots in each frame are assigned to the same user. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Each logical channel is assigned a time slot or slots on a common carrier frequency. The radio transmissions carrying the user communications over each logical channel are thus discontinuous. The radio transmitter is off during the time slots not allocated to it.
Each separate radio transmission, which should occupy a single time slot, is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, one for the initial access and synchronization of a user to the system, and another one for routine communications once a user has been synchronized. Strict timing must be maintained in TDMA systems to prevent the bursts from one logical channel from overlapping and hence interfering with the bursts from other logical channels in the adjacent time slots.
Space Diversity.
Space diversity is a method for improving signal quality by the use of multiple spaced-apart transmitting and receiving antennas to send forward channel signals or receive reverse channel signals from a single receiver/transmitter. On the forward link, signals from multiple spaced-apart transmit antennas are received by a single receiver. On the reverse link, multiple spaced-apart receiving antennas receive signals from a single transmitter. Micro-diversity is one form of space diversity that exists when the multiple transmitting or receiving antennas are located in close proximity to each other (within a distance of several meters for example). Micro-diversity is effective against Rayleigh or Rician fading. The terminology micro-diverse locations means, therefore, the locations of antennas that are close together and that are only separated enough to be effective against Rayleigh or Rician fading. The signal processing for micro-diverse locations can occur at a single physical location and micro-diversity processing need not adversely impact reverse channel backhaul bandwidth requirements.
Macro-diversity is another form of space diversity that exists when two or more transmitting or receiving antennas are located far apart from each other (at a distance much greater than several meters, for example, hundreds of meters or several kilometers). The terminology macro-diversity means that the antennas are far enough apart to have decorrelation at the receivers between the mean signal levels. In macro-diversity systems, on the forward channel the transmitted signals from the multiple transmitter antennas are received by the single receiver and processed to form an improved resultant signal at that single receiver. On the reverse channel, the received signals from the single transmitter are processed and combined to form an improved resultant signal from that single transmitter. On the forward channel, the decorrelation is between the mean signal levels for the multiple transmitted signals received by the single receiver. On the reverse channel, the decorrelation is between the mean signal levels for the multiple received signals from the single transmitter. On the reverse link, since macro-diversity processing involves forwarding of signals to a common processing location, an adverse impact on backhaul channel bandwidth tends to result from macro-diversity processing.
In the cross-referenced application, a cellular system is shown having a zone manager that broadcasts forward channel (FC) communications from a broadcaster to multiple users located within a zone. The broadcaster, in the embodiment described, is distributed to include distributed broadcaster transmitters and a broadcaster control 14. The broadcaster transmitters are sited at macro-diverse locations relative to each other within the zone. One or more of the broadcaster transmitters broadcasts in a forward channel (FC) to each of the users. The broadcaster control includes a broadcaster signal processor receiving control signals and operating to control the signals that are broadcast on each of the broadcaster transmitters. The control signals are derived from the reverse channel signals from collectors. Each of the multiple users transmits reverse channel (RC) communications to one or more of multiple collectors, which in turn forward the reverse channel communications to an aggregator in the zone manager. The aggregator in turn provides the control signals to the broadcaster control for signal processing to assist the broadcaster control to select and control the forward channel signals.
In the cross-referenced application, each of the users has a receiver antenna for receiving broadcasts on the forward channel from one or more of the broadcaster transmitters of the broadcaster. Also, each of the users has a transmitter that transmits on a reverse channel to the collectors. The collectors are sited at macro-diverse locations relative to each other within the zone. Therefore, multiple copies of macro-diverse reverse channel communications are received at the aggregator for each user.
In the cross-referenced application, a typical user, U1, has forward channel (FC) communications (bc/btFC) from the broadcaster control to each of the broadcaster transmitters and forward channel communications (bt/uFC) from each of the broadcaster transmitters to user U1. The user U1 has user-to-collector reverse channel communications (u/cRC) to each of the collectors, and the collector-to-aggregator reverse channel communications (c/aRC) for each of the collectors to aggregator 17.
The forward and reverse channel communications of the cross-referenced system apply to any digital radio signal system including for example TDMA, CDMA, SDMA and FDMA systems. If the digital radio signals of any particular system are not inherently burst structured, then arbitrary burst partitions may be used for confidence metric processing.
Shadow Fading.
The decorrelation of mean signal levels employed in macro-diversity systems is due to local variability in the value of signal strength diminution between the single receiver/transmitter and the spaced apart transmitting and receiving antennas. This local variability exists on length scales above Rayleigh or Rician fading and is due to terrain effects, signal blocking by structures or vegetation, and any other variability that exists in a particular environment. This variability is referred to as shadow fading. Decorrelation lengths for shadow fading may be as small as length scales just above Rayleigh fading length scales (for example, less than a few meters), or may be as large as several kilometers.
Channel Allocation.
Many communications systems multiplex multiple users on a data channel in order to increase system efficiency. In such systems, a common control channel is often required to maintain allocations of the users sharing that resource. The control channel must be detected and analyzed by all the users sharing a resource. This control channel is often embedded in the data channel itself.
In addition, since mobile systems operate in very harsh radio environments, extensive measures are taken to enhance the performance of the radio link between the base station and the users. Many of these techniques require the optimization of the downlink for a specific user, at the expense of other users. An example of such a technique is downlink power control. Unfortunately, when a downlink channel is optimized for one user, the other users wishing to detect the control information, embedded in the data channel, quite possibly cannot detect it. This operation can result in detrimental consequences such as missed allocations, thereby reducing capacity.
A number of the downlink optimization techniques suffer from these harmful consequences, for example, packet data based GPRS/EGPRS or EDGE systems. Such systems have been developed to coexist with GSM systems which are the most widely deployed digital mobile communication systems in the world.
In multiple access communication systems, a shared resource, which is typically a communication channel, must be assigned to a specific communication unit (CU) for a specific duration in order to use the resource efficiently. Moreover, the assignment of the resource must be known to all other potential users of the resource in order to eliminate the possibility of multiple CU's colliding on that channel. The channel allocation procedure used to perform this assignment must usually meet certain significant performance requirements such as minimal assignment delay, efficient usage of the channel and quick response to changing channel conditions while incurring the least overhead in terms of inter-CU signaling.
Fixed allocation of channels eliminates contention for channels by assigning the channel usage to a specific CU. All other CU's receive similar assignments and are prohibited from accessing the channel when it is not assigned to them. Another approach to addressing the problem is by use of dynamic channel allocation where all CU's are required to monitor a common channel, referred to as an allocation block or allocation channel, which contains the assignment of the channel until the next instance of the allocation block. Dynamic channel allocation permits better utilization of the channel than fixed allocation since channel assignment can be varied continuously depending on the offered load and the channel conditions. In a wireless system (such as GPRS/EGPRS or EDGE), the channel properties can change dramatically over short periods of time and hence the ability to control and assign channels dynamically can significantly improve channel utilization.
In a GPRS/EGPRS or EDGE system, dynamic channel allocation is performed by transmitting a Uplink Status Flag (USF) on each downlink radio block. This USF is used to reserve a set of one or more subsequent uplink radio block(s) for a specific user (mobile station or MS) from among a set of MS's sharing the uplink packet data channel (PDCH).
Downlink Power Control.
The downlink radio block carrying the USF may be destined for an MS (for example, MS-A) different from the MS (for example, MS-B) for which the USF is targeted. In this scenario, down link power control must be performed in such a way that the radio block can be decoded properly by the MS-A as well as MS-B. This results in potentially severe restrictions on the power control algorithm.
Fast Macro-Diverse Switching (FMS).
The use of downlink macrodiversity (MD) in an FMS system results in a similar problem which manifests when a remote transmit (TX) resource is being used to transmit a radio block (for MS-A) containing a USF intended for MS-B (which might be best served from a local TX). The problem then is one of ensuring that MS-B is able to properly receive and decode the USF on a radio block for MS-A. This problem can place severe restrictions on selection of the best TX resource for FMS.
Smart Antennas.
The use of antenna arrays with beams directed towards specific regions within a cell causes the same problem as described for FMS systems. Since smart antenna systems rely on finding the best beam or antenna for the intended receiving MS (MS-A), MS-B may not be able to receive and decode the USF properly unless MS-B happens to be located within the region covered by the beam/antenna used for MS-A. Consequently, either depending on the relative positions of MS-A and MS-B, a wider beam must be used nullifying the benefits or severely impairing the efficacy of the smart antenna system, or the choice of MS's that can share the uplink resource must be severely restricted.
In accordance with the above background, the communications problems resulting from interference, noise, fading and other disturbances create a need for improved wireless communication systems which overcome the interference problems and other limitations of conventional cellular systems.